The proposed course of this project for the next fiscal year is: (1) The growth of ecotropic and xenotropic viruses in cells of different species will be studied further, with particular interest in the effect(s) that long term cultivation in heterologous cells might have on the host range of these viruses. Mechanism(s) controlling resistance and permissiveness by different cells to these two types of viruses will also be investigated. Efforts to resolve the relatedness of PRLV to ecotropic and xenotropic viruses will be pursued using immunological and biochemical techniques. The ongoing in vivo studies will be continued. (2) Further studies on the biology of H. saimiri will be pursued. Mutants with reduced capacity for cytopathology or growth at low temperatures (temperature sensitivity) will be isolated and tested for cell transforming potential, oncogenicity, and possible usage as vaccines. In vivo studies will be continued. (3) The epitheloid cell clone will be further characterized for possible mammary origin by testing for hormonal effects on cell growth and casein production. The biology of the type C virus produced by these cells will also be studied.